I-II-VI2 group chalcopyrite compound semiconductors, which can be represented by CuInSe2, have a direct transition energy band gap, very high light absorption which makes it possible to produce highly efficient solar cells in the form of a thin film having a thickness of several micrometers, and excellent electrical and optical stability, thereby being very ideal for a light absorber material of solar cells. Specifically, Cu(In,Ga)Se2 solar cells have been emerging as those being capable of replacing the conventional expensive crystalline silicon solar cells, owing to their highest energy efficiency (NREL, >19%) among thin film solar cells, and high price competitiveness as compared to conventional silicon-based solar cells. However, multinary compounds such as chalcopyrite compounds require very complex manufacturing processes. Therefore, there are still needs for constant reduction of production cost through process improvement in solar cells based on chalcopyrite compounds in order to compete with fossil fuel.
Since CuInSe2 compounds have 1.04 eV of an energy band gap just right below the band gap (1.4 eV) ideal for a solar cell, solar cells based on them have relatively high short-circuit current (Jsc), but relatively low open-circuit voltage (Voc). For raising the open-circuit voltage, a part of indium (In) is often substituted by gallium (Ga), or a part of selenium (Se) with sulfur (S). These are, depending on the components, expressed as follows: CuInSe2 (CIS), CuGaSe2 (CGS), Cu(In,Ga)Se2 (CIGS), CuInS2, CuGaS2, Cu(In,Ga)S2, CuIn(Se,S)2 (CISS), CuGa(Se,S)2 (CGSS) and Cu(In,Ga)(Se,S)2 (CIGSS), and these compounds are collectively referred as CIS solar cells. CIS solar cells are generally formed by sequentially applying 5 layers of thin film units—a back-contact electrode, a light absorber layer, a buffering layer, a front transparent electrode and an anti-reflective coat on a substrate, which is generally glass. For each thin film unit, various species and compositions, and for manufacturing methods, various kinds of physical or chemical thin film manufacturing processes can be used.
The light absorber layers in CIS solar cells are typically fabricated by physical evaporation methods using vacuum techniques such as co-evaporation, sputtering and the like. The co-evaporation, a technique of fabricating a thin film by feeding each element such as Cu, In, Ga and Se into a small electric furnace installed in a vacuum chamber, and heating them for vacuum evaporation on a substrate, was taken by a National Renewable Energy Laboratory (NREL) in US, resulting in CIGS solar cells having 19.5% energy conversion efficiency. However, this method requires great investment at early stage and is difficult to expand the scale since it uses a high vacuum technique. Further, serious contamination inside the vacuum equipment makes it hard to fabricate a thin film in continuous and reproducible way. Sputtering is a method widely used in this field since it requires a rather simple set-up and can easily evaporate metal or insulating materials. For example, Shell Solar Company manufactures a CIGS thin film by sequentially sputtering a copper-gallium alloy target and an indium target so as to fabricate a thin film of Cu—Ga—In alloy and treating it with heat in H2Se gas atmosphere. This method is advantageous in that manufacture is relatively easier than that using co-evaporation, but still requires great investment at early stage and also has limit on expanding the scale since it is also based on a vacuum technique.
Co-evaporation and sputtering make it relatively hard to produce large-area solar cells, and are one of main causes for lowering the price competitiveness of solar cells owing to the high production cost. In order to find alternative methods to such problems, researches on nanoparticle processes are being currently made, wherein light absorber layers are manufactured by depositing nanoparticles onto a substrate through a methods such as spraying, screen printing, inkjet printing, doctorblade, spin casting and the like, other than physical vapor deposition using vacuum technology, and treating the resulted substrate with heat. FIG. 1 is a view schematically illustrating a method for fabricating a light absorber layer by using a nanoparticle process. According to the nanoparticle process, a thin film of light absorber layers is formed by coating nanoparticles (101) manufactured as disclosed in FIG. 1 on a substrate (103) and heating the nanoparticles to form a polycrystalline thin film (102).
For manufacturing the light absorber layer for a solar cell by using a nanoprocess, synthesis of nanoparticle precursors comprising the corresponding elements should be carried out first. The precursors are largely classified by nanoparticle oxides of binary or ternary compounds such as CIS, CIGS nanoparticles, Cu—In—O, Cu—In—Ga—O and the like.
U.S. Pat. No. 6,268,014 reports a technique which comprises: synthesizing an oxide of a ternary compound (e.g. Cu2In1.5G0.5O5, Cu2In2O5 Cu2O—In2O3) in less than micrometer scale by an ultrasonic nebulizer; forming the resulted product as a solution or paste to form a thin film; heat-treating the thin film in a reducing atmosphere, resulting in a CIS thin film. This method has a problem such that the average size of the particles are rather big as being in the range of hundreds nanometers, being unable to lower the heating process temperature. As a similar method, EP patent No. 0 978 882 A2 discloses a method of fabricating a CIS thin film which comprises: synthesizing oxides of Cu and In by precipitating Cu-hydroxide and In-hydroxide from an aqueous solution and heating them; depositing the oxides on a substrate to form a thin film; heat-treating the thin film in a reducing atmosphere. This invention also results in the particles in micrometer scale, not nanometer scale, and the heating temperature is about 550° C. Further, the invention requires an additional step of supplying selenium after removing oxygen from the oxides, since it employs oxides as a precursor.
U.S. Pat. No. 6,126,740 reports that it is possible to obtain CIGS thin film by reacting CuI, InI3 and GaI3 dissolved in pyridine with Na2Se dissolved in methanol at a low temperature to obtain a CIGS colloid, then depositing the colloid onto a substrate by, for example using spray deposition, and then heating the resulted layer. However, this method has difficulties such that pretreatments for deoxidization and dehydration are required and the whole process should be performed in inert atmosphere. Yitai Quian et al. (Adv. Mater. 11(17), 1456-1459 (1999)) has reported the synthesis of CIS nanoparticles by subjecting CuCl2, InCl3 and Se particles in a mixed solvent of ethylenediamine and diethylamine into a solvothermal route. This method also has problems such that manufacture and separation of precursors are difficult owing to the use of a toxic strong basic solvent, amine compounds, and it requires a long reaction period of one day or more and a high reaction temperature of 180° C. or more.